1. Field of the Invention
The present invention generally relates to the separation of a wide band electrical input signal into individual frequency channels, and more specifically to an electro-optic modulator which may be advantageously incorporated into a wide band channelized receiver.
2. Description of the Related Art
Electronic spectrum analyzers and channelized receivers or demultiplexers are similar in principle in that they separate a wide band input signal into individual channels corresponding to discrete frequency ranges within the bandwidth of the input signal. A simplified electronic equivalent of such a device would consist of a plurality of bandpass filters connected in parallel, each designed to pass frequency components of the input signal within a particular frequency range therethrough. Spectrum analyzers are generally constructed to produce a plot of signal amplitude vs. frequency on a cathode ray tube or other display device. Channelized receivers or demultiplexers are used to separate a wideband composite input signal including carriers of different frequencies which are individually amplitude or otherwise modulated with information into separate electronic channels.
Although the separation of an input signal into individual frequency components or channels is relatively straightforward in principle, a variety of diverse methods of accomplishing this function in actual practice have been proposed in the prior art. Superheterodyne receiver circuits, which form the basis for most broadcast radio receivers, have been adapted to sweep across a specified frequency band and intercept signals at individual frequencies which may exist in the band. However, this method is only capable of detecting one frequency at a time, and the intercept probability can be very low for intermittent signals. Compressive filters may be incorporated into superheterodyne spectrum analyzers to increase the intercept probability.
The crystal video spectrum analyzer is the modern version of the crystal set, and includes a broadband filter and crystal detector which detects any signal within the input filter width. These analyzers have poor sensitivity and frequency resolution, and do not provide any frequency information in their basic form. However, discriminator techniques may be incorporated into the design to provide a single frequency reading.
The receivers discussed above are basically single channel devices. Multi-channel receivers, which more closely relate to the present invention, have been developed which function in accordance with the parallel bandpass filter configuration discussed above. These receivers enable simultaneous reception of any number of frequency ranges with 100% intercept probability, and provide a separate reception channel corresponding to each frequency range. The obvious extension of this approach to simultaneous, multi-channel reception is to physically provide a bandpass filter for each channel. This is satisfactory where the number is channels is small, but impractical where separation of a large number of channels is required.
Channel separation may be advantageously performed by means of an acousto-optic modulator (AOM). This device and a channelized receiver in which it is incorporated is discussed in a textbook entitled "Acoustic-Optic Signal Processing", by N. Berg et al, Marcel Dekker, Inc., Chapt. 4, pp. 87-106. A presentation of the single channel analyzers discussed above is also found in this reference. The acousto-optic channelized receiver (AOCR) is also presented in a paper entitled "Acousto-optic channelized receivers", by P. Kellman et al, Optical Engineering, vol. 23, no. 1, (Jan./Feb. 1984).
In an acousto-optic receiver, a transparent ultrasonic delay line (Bragg cell) is utilized to convert a wideband electrical input signal into a proportional optical pattern by means of a travelling pressure wave. Spatial variation of the refractive index is used to modulate a coherent light beam from a laser, and the diffracted spectral components are separated by a lens. Fourier transformation of the input signal by the lens produces a light distribution in the focal plane of the lens. This light distribution is detected photoelectrically, producing a charge distribution proportional to the instantaneous power spectrum of the input signal. An array of photodetectors is used to generate charge at discrete positions.
An AOM functions by diffracting light at an angle which is determined by the spatial period and propagation direction of the travelling pressure waves and with an intensity proportional to the signal power. Although this device is capable of channelizing an input signal over a wide bandwidth using a relatively simple structure, its application is rather limited.
More specifically, the fixed acoustic velocity in an acousto-optic cell is an impediment to fine frequency resolution and programmability. The input signal is impressed on a carrier, and used to excite sound waves in a crystal. A coherent light beam incident on the crystal at the Bragg angle is diffracted by the sound waves and Fourier transformed by the lens. The spatial Fourier transform is integrated on a channelized detector array. The separation between adjacent channels on the detector array, or equivalently the frequency resolution of the acousto-optic cell, is limited by the transit time of acoustic waves across the cell aperture. The slowest available acoustic velocity is that for the slow shear wave in TeO.sub.2, which is 6.2.times.10.sup.4 cm/sec. For an exemplary 2 cm aperture, this corresponds to a frequency resolution of 31 KHz, which is insufficient to provide the necessary adjacent channel rejection ratio in many applications. Also, since the acoustic velocity is fixed, the channel widths cannot be changed without physically changing the detector or lens. Other problems inherent in AOMs involve distortion due to spatial attenuation of the acoustic waves, and critical alignment requirements.
Another type of channelized receiver is known as a Surface Acoustic Wave (SAW) receiver, and is presented in a paper entitled "SAW RF SPECTRUM ANALYZER/CHANNELIZER USING ACOUSTIC WAVE DIFFRACTION", by R. Brooks, SPIE vol. 639 Optical Information Processing II, pp. 154-159 (1986). In this device, an RF signal to be analyzed drives a phased array of SAW interdigital transducers which acts like a curved diffraction grating to focus and angularly disperse the generated acoustic waves with frequency. An array of output transducers partitions the dispersed signal spectrum into contiguous narrow bands.
The SAW device differs from the AOM in that no light is involved, and the propagating waves are generated directly from the input signal, rather than by modulating waves from an external source. However, SAW modulators have the same drawbacks as AOMs in that they are limited in flexibility and programmability.
Yet another type of channelized receiver is disclosed in U.S. Pat. No. 4,468,766, issued Aug. 28, 1984, entitled "OPTICAL RF DOWNCONVERTER", to A. Spezio. This reference teaches how to separate a wide bandwidth optical signal into frequency channels utilizing a set of diffraction gratings.